Nadsat is a mode of speech used by the nadsat, members of the teen subculture in the novel A Clockwork Orange. The anti-hero and narrator of the book, Alex, uses it, in first-person style, to relate the story to the reader. He also uses it to communicate with other characters in the novel, such as his droogs, parents, victims, and any authority-figures he comes into contact with. It is not a written language: the sense that we have of the novel is of a transcription of vernacular speech, rather than an implementation of a published, bona-fide dialect.

Nadsat is basically English, with some transliterated words from Russian. It also contains influences from Cockney rhyming slang and the King James Bible, some words of unclear origin, and some that Burgess invented. The word nadsat itself is the suffix of Russian numerals from 11 to 19 (-надцать). The suffix slurs the Russian words for 'on ten'—for example, 'one-on-ten,' 'two-on-ten,' (одиннадцать, двенадцать) and so on—and thus forms an almost exact linguistic parallel to the English '-teen.' Some of the words are also almost childish English such as eggiweg (egg) and appy polly loggy (apology), as well as regular English slang sod and snuff it. The word like is often inserted arbitrarily into phrases.